The present invention relates to the measuring of noise or other sound and more specifically to a process for measuring noise or other sound for recording noise (sound) volumes above at least one selectable sound level, the noise being sensed and converted to an electric signal. Furthermore, the invention relates to a device for putting the process into effect.
Noise occurs in many places of work, particularly in industry, and is of a highly varying character both as regards intensity, duration and frequency distribution. Thus, noise may be permanent, intermittent or fluctuating in character, or may be in the nature of pulses.
Since it has been proved that too much noise will damage the human auditory organs and may also to some extent reduce hearing, there is a great need for measuring and recording noise in order to map sources of noise and above all in order to see to it that persons who work or live in noisy environments are not exposed to too much noise during too long a time.
According to the Swedish regulation (norm SEN 590111) the risk that a person will have his hearing damaged is depending on the total exposure or noise doses, i.e. the quantity of sound energy or noise, to which the person in question is exposed during one day. The noise dose is consequently a product of the noise intensity (sound energy) and the time of its duration (exposure time). Since this norm can only be applied strictly when the noise or sound level is constant, the norm also indicates a modified method of measuring the total exposure time for each occurring sound level (noise distribution analysis). By means of this additional method it is possible to evaluate the total noise quantities obtained for the sound levels in question in order to obtain a permanent average sound level corresponding from a risk viewpoint to the measured varying sound level during the same time. This value is called the equivalent continuous sound level and is expressed in dB(A).
Prior-art means for measuring noise exist of both the indicating and the recording type, and they may be stationary or portable. Preferably, such means are utilized as may be easily carried by the person who is the object of the noise measuring. For this purpose it is required that the device is so small that it will not hinder the person's normal activity.
Prior-art noise measuring means of the indicating type are provided with lamps or other indicating means, which are activated in dependence on the sound level reached at the measuring means in question. Means of the indicating type can only be used in order to indicate the sound level of the noise at the place where the measuring takes place and will only give a vague notion of how the sound level of the noise varies with the time, and do not indicate any recording of the measured noise quantity in relation to time.
Generally occurring noise measuring means of the recording type are so called noise dosimeters, of which there are several embodiments, preferably of a size which will fit in a breast pocket and having a noise sensing microphone arranged in such a way that it can be suspended near the carrier's ear. A common characteristic of these noise dosimeters is that they are based on a measuring of the noise dose, i.e. the quantity of noise to which a carrier is exposed during a determined time, for instance one day or one week. In that case the noise dose is measured by means of a summation process, viz. as the sound intensity multiplied by time, in which case an equivalent sound level is calculated by means of differential calculating means according to the formula: ##EQU1## L indicates the equivalent sound level in dB(A) T indicates the measuring time
I indicates the intensity of the noise in dB(A).
The recording of the calculated value is then carried out by means of various kinds of counters or counting mechanisms.
The measuring and recording of noise using dosimeters is a simplication and a rough approximation, since it does not give any possiblity of studying subsequently to what extent the recorded noise relates to different sound levels and consequently does not admit of any noise distribution analysis. According to SEN 590111 it is further indicated that a pulse sound having a sound level below 140 dB and having a duration of less than 25 msec will not be detrimental or dangerous to hearing. However, repeated pulse sounds, even if they are of a shorter duration than 25 msec, may no doubt cause damage to hearing.
A drawback in existing noise dosimeters is the time delay caused by the nature of the components of the dosimeters, which entails that sounds having a duration of less than 200 to 500 msec, e.g. hammer blows, cannot be recorded.
Another drawback of existing noise dosimeters is that the time duration, i.e. the time during which the measuring takes place, cannot be recorded but must be noted separately.
Yet another drawback of noise dosimeters is that they cannot be used internationally since the norms for the risks of hearing damage vary from country to country.
Thus, in Sweden it is assumed that a certain quantity of sound energy (noise dose = intensity multiplied by time) is equally detrimental to hearing, irrespectively of how long it lasts in time. This principle is called the equal energy principle and implies that if the noise lasts half the time, the sound intensity is allowed to be 3 dB higher. From this follows the concept q = 3. Thus, a noise dosimeter in Sweden should be constructed in order to give measuring results equivalent to q = 3. In the United States the norm applied is q = 5, which means an increase of 5 dB at the halving of the exposure time. There are also dosimeters recording sound pressure multiplied by time instead of sound energy multiplied by time. For such dosimeters q is equal to 6, and values obtained by these dosimeters must therefore be corrected when a comparison is made with dosimeters for which q is equal to 3. Obviously, therefore, there are considerable drawbacks in comparison with measuring results obtained by means of noise dosimeters.
There are, however, devices for measuring and recording noise which are independent of the q-norms of different countries and which may therefore be used internationally in order to calculate noise which will damage the hearing. These devices, which are meant for noise distribution and analysis, will store the time duration of the noise in different sound level classes and will thus give considerably nore information on the character of the noise, since it is possible to decide subsequently directly which sound level or levels have contributed most to the noise dose received.
Prior-art means for noise analysis are however highly complicated and consequently bulky. They are not, therefore, suitable for mobile measurings and particularly not for being carried by a person who goes about his normal daily duties.
The purpose of the present invention is to provide a process for measuring noise for recording said noise in order to make a noise distribution analysis at selectable sound levels, which process does not have the drawbacks mentioned above but is suitable for use at mobile measurings.
An additional purpose of the invention is to provide a device for putting the process into effect which is simple and inexpensive and which can be carried in a simple manner by a person in such a way as to give a correct measuring result without hindering the ordinary activity of the said person in any way whatever.